Other than Chaengwattana there is no official immigration office that will do 30 day extensions on 30 day visa exempt or 60 day tourist visas.
Bumrungrad hospital has a service where an immigration officer comes there on Saturday from 9-11 and does 30 day extensions for the 1900baht plus an 800 baht service charge.
You should call and put your name on the list, because people who just showed up were told to do so next time or they couldn't get an extension.
**********************************************************************************